1. Field of the Invention
The present invention generally relates to printed circuit board (PCB) technologies and, more particularly to PCBs having reduced mounting height.
2. Discussion of the Background
An assembly process of semiconductor devices starts with isolation of individual chips from a wafer. Each of the isolated chips is electrically interconnected to a lead frame and encapsulated by a molding compound (package body) for protecting the electrical interconnection and the chip from harmful external environment. The semiconductor chip under the assembly process is referred to as “semiconductor chip device”, while the packaged chip after the assembly process is referred to as “semiconductor package device”.
The semiconductor package device has various mounting structures on a circuit board (PCB), which includes dual in-line package (DIP), pin grid array package (PGA), J-shaped bent outer leads, gull-wing, straight lead, pin grid array (PGA) package, and ball grid array (BGA) package structures. The BGA package provides a relatively high surface-mount density and improved electrical performance when compared with the conventional plastic package structures employing the metal lead frame.
FIG. 1 is a cross-sectional view illustrating mounting structure of the conventional BGA package 20 on a PCB 10.
A primary difference between the BGA package 20 and a conventional plastic package is that the electrical connection between the package device and the PCB 10 is provided by a plurality of solder balls 22. The solder balls 22 are attached to the bottom surface of the package device 20 in an array pattern so that the area occupied by the BGA package on the PCB 10 is smaller than that of a conventional plastic package with peripheral terminals. That is, the BGA package device 20 is surface-mounted onto the PCB 10 that includes a plurality of solder ball pads 12 and wiring patterns 14.
In terms of mounting density in vertical direction, the mounting height of the BGA package on the PCB is determined by two factors: the thickness of the package 20; and the diameter of solder balls 22. Therefore, if the mounting structure of BGA package is fundamentally modified from the conventional one, dramatic reduction in the mounting height is possible.